Like Earth, Mars has a tilted axis. But because Mars takes longer to circle the sun, its seasons are nearly twice as long.

With the sun hanging low and appearing only for a brief time each day, keeping Opportunity charged via its solar panels is a challenge. Engineers worried the rover wouldn't make it through its first winter on Mars. But this week, the rover emerged on the other side of the season's shortest-daylight weeks.

"Opportunity has made it through the worst part of its eighth Martian winter," Jennifer Herman, power subsystem operations team lead for Opportunity at NASA's Jet Propulsion Laboratory, said in a news release.

To keep Opportunity charged and rolling through the fall and winter in Mars' southern hemisphere, engineers tilt its solar arrays northward toward the equator.

The rover is currently exploring Perseverance Valley, a region carved by ancient liquid flows. Scientists have plotted a course through the valley that allows the rover to stop and study at north-facing sites, allowing the rover to power-up as it carries out its scientific duties.

Engineers must pay close attention to the rover's energy levels to make sure they don't tax its batteries.

"Relying on solar energy for Opportunity keeps us constantly aware of the season on Mars and the terrain that the rover is on, more than for Curiosity," Herman said.

Even with planning, things don't always work out. A few years ago, Opportunity was forced to spend 19 weeks at a single north-facing location, as no other suitable sunny spots were within a day's drive.

Scientists also have to be cognizant of other factors that can interfere with the rover's ability to charge. While fall and winter typically feature clear skies, dust storms can dirty the rover's solar arrays as autumn approaches, limiting its charging abilities. Serendipitous gusts, however, can help clean off the panels.

"We were worried that the dust accumulation this winter would be similar to some of the worst winters we've had, and that we might come out of the winter with a very dusty array, but we've had some recent dust cleaning that was nice to see," Herman said. "Now I'm more optimistic."

But even in spring and summer, when the sun rises higher and stays out longer, the threat of a major sun-blocking dust storm looms. The last major storm in 2007 forced scientists to limit the rover's scientific activities.

Some planetary scientists think Mars is due for another big dust storm in the southern hemisphere in the spring of 2018.

"If Opportunity's solar arrays keep getting cleaned as they have recently, she'll be in a good position to survive a major dust storm," Herman said. "It's been more than 10 Earth years since the last one and we need to be vigilant."